Our present invention relates to an apparatus for applying a coating composition comprised of a liquid resin and fine-grained abrasive material, especially corundum, to a continuous paper web which can be for the production of wear-resistant laminates.
German patent document DE 195 08 797 C1 describes a process for the production of decorative paper which can be used to make wear-resistant laminates. In this system, a viscous mixture of a melamine resin and alpha-cellulose, to which corundum articles are added, is coated as the coating composition onto the visible surfaces of a decorative paper. The alpha-cellulose there serves as a stiffening and suspending agent. The corundum particles have a particle size of 15 to 50 xcexcm. The coating composition is applied in an amount of 80 to 200 g/m2 so that after the composition has been dried to its final moisture content, a layer thickens to 20 to 65 xcexcm is obtained. The coating composition os applied by a wire doctor, a reverse coating system or a screen roller.
In German patent document DE 198 14 212 C1, an apparatus has been described which is especially designed for the application of such coating compositions. This apparatus has been successfully in use for some time and can be considered to be a basic coating system over which the present invention is an improvement. With this apparatus, a rerouting or guide roller around which the paper web is looped, is juxtaposed with a metering roller and the metering roller and the guide roller and so synchronized that in the gap between the metering roller and the paper web, there is no relative motion between the web and the coating composition. The peripheral speeds of the rollers and of the paper web, the gap dimension and the directions of movement of the surfaces defining the gap are thus selected to prevent such relative movement in the gap.
In this manner, the wear of the metering roller can be minimized in spite of the fact that the coating composition contains abrasive particles. This system, however, has a problem which may originate in the fact that the two rollers have the same peripheral speed at the gap and that is that, unlike the case where the rollers run in opposite senses, part of the coating composition adheres to the web while another part remains adherent to the metering roller and is drawn away from the web at the downstream side of the gap.
In practice it is found, in such a system, that the layer which adheres to the web is somewhat nonuniform and may have clearly visible longitudinal stripes.
To eliminate this striped pattern, an equalizing system can be provided along the web downstream of the gap. The stripes can be eliminated for example by a doctor blade, e.g. comprised of a substantially rectangular flap of a flexible rubber-like material which is secured on a support bar parallel to the axes of both rollers at a rear edge of the doctor blade and which rests loosely on the coated upper pass of the paper web as it is delivered by the guide or rerouting roller.
Depending upon the consistency of the coating mass which can be a function of the composition selected by the producers of the decorative paper and which may be required for different laminating purposes, such a doctor flap can be more or less effective in removing the undesired strip pattern. In some cases the striped pattern cannot be removed by the use of such a flap or doctor member alone.
U.S. Pat. No. 19 39 012 describes the use of a worm for smoothing the upper surface of moist molded tile blanks. A mold filled with moist clay is passed below the rotating worm which strips off excess clay. The excess clay passes laterally into a shaft where it is collected for reuse.
German patent document DE 195 41 000 describes an apparatus for smoothing continuous plaster fiberboard and comprises substantially of a worm disposed transversely above the conveyor belt for the continuous plaster fiberboards. The rotating worm is formed with a kind of shaver for the surface of the continuously-passing and already set plasterboards. The latter can be either still moist or even dried and the worm removes fine particles which are transported by the worm to the edge of the board and are collected in a container.
It is, therefore, the principal object of the present invention to provide an apparatus for coating paper webs for the purposes described with an abrasive coating composition and particularly coating compositions containing a resin and corundum particles, whereby the drawbacks of the systems previously described are obviated.
Another object of this invention is to provide an apparatus for coating paper webs in the production of decorative paper with thick coatings containing fine-grain adhesive which ensures uniform coating with coatings which are free from striped patterns characterizing earlier systems and which is not dependent upon the particular coating composition that is the case with some prior coating systems.
These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, in an apparatus for coating paper webs with a composition as described and which comprises:
a guide roller around which the web is looped so that the web has a coating-receiving surface extending arcuately around the guide roller;
a metering roller parallel to the guide roller and having a surface juxtaposed with the surface of the web across a narrow gap;
a slit nozzle juxtaposed with one of the surfaces for delivering the composition over a width of the web, the composition passing through the gap and forming a coating on the coating-receiving surface of the web; and
an equalizing device for rendering the coating uniform, the equalizing device including at least one structured roller juxtaposed with the coating-receiving surface downstream of the gap and having a screwthread-like structure.
According to a feature of the invention, the screwthread-like structure can be continuous, i.e. a helical rib extending in one or more turns along the structured roller or discontinuous, i.e. a series of lands or formations arrayed in a helical pattern. The screwthread-like structure can form a single-thread structure or a multiple-thread structure and can be a system of mutually-crossing helical grooves of opposite hand or rotational sense.
According to a feature of the invention, the equalizing device further comprises a doctor blade engaging the coating and located upstream or downstream of the structured roller and in a form of a substantially rectangular flap of a flexible elastomeric material having a rearward edge parallel to the structured roller and fastened to a support and lying slackly upon an upper pass of the web coming off the guide roller.
The equalizing device can further comprise a smooth roller in contact with the coatings and downstream of the structure roller in a direction of travel of the web.
According to another feature of the invention the equalizing device includes at least two parallel structured rollers juxtaposed with the coating-receiving surface downstream of the gap and each having a screwthread-like structure.
It has been found to be advantageous to provide supports for the web close to the equalizing device and upstream and downstream therefrom in a direction of travel of the web. The guide or rerouting roller can form one of these supports while another of these supports is formed by a support roller downstream of the equalization device. The metering roller can be located at least in part above the guide roller at a location in which the web is delivered horizontally from the guide roller and in that case the metering roller can be located vertically above or approximately vertically above the guide roller. With the invention, moreover, a doctor blade can be juxtaposed with the metering roller.
With the system utilizing one or more structured rollers and, if desired, doctor blades or smooth-surfaced rollers downstream thereof in the equalizing device, we are able to ensure stripe-free and uniform coating with practically all compositions containing liquid resins and abrasive particles like corundum on paper web.